1. Field of the Invention
The present invention relates to radio apparatuses, and it particularly relates to a radio apparatus using multiple subcarriers and a communication system utilizing said radio apparatus.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been applied to the wireless standards such as IEEE802.11a/g and HIPERLAN/2. The packet signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carries in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the burst signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in the packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining such a MIMO system with the OFDM modulation scheme results in a higher data transmission rate. For the purpose of enhancing the transmission efficiency in this MIMO system, the data signals to be transmitted respectively in a plurality of packet signals are aggregated into a single packet signal. In so doing, the control signals are appended to the respective data signals. In other words, a plurality of combinations of control signals (hereinafter referred to as “MIMO control signals”) and data signals are contained in the packet signals. In the case when the receiving apparatus receives these control signals and data signals, the weights and channel characteristics need to have been derived beforehand. Accordingly, the known signal in a MIMO system (hereinafter referred to as “MIMO known signals”) are contained in the packet signals.
The compatibility with a system (hereinafter referred to as “legacy system”), which is not a MIMO system, is also required. In other words, it is required that the existence of packet signals be let known to a receiving apparatus of a legacy system. Accordingly, a known signal and a control signal for a legacy system (referred to as “legacy known signal” and “legacy control signal”, respectively) are assigned to a header portion of a packet signal. If these are aggregated, the packet format will be composed of a legacy known signal, a legacy control signal, a MIMO control signal and a MIMO known signal both contained in a leading combination, a data signal contained in the leading combination and the remaining combinations. Here, in the remaining combinations, the signals are arranged sequentially in the order of a MIMO control signal and a MIMO data signal.
It is generally the case that the amount of information to be transmitted by the legacy control signal and the MIMO control signal (hereinafter these will be generically referred to as “control signal”) is smaller than the amount of information to be transmitted by the data signal. Thus, it is possible that the control signal is transmitted by a single stream. However, if the control signal is assigned to a single stream but the signals other than the control signal are assigned to multiple streams, the power will differ only in part of the packet signal. To reduce such power fluctuation, the signals in which the control signal is subjected to cyclic timing shift in the period of control signal are assigned to the remaining streams. Such processing is generally called CDD (Cyclic Delay Diversity). To conform to the control signal, the legacy known signal and the MIMO known signal (hereinafter these will be generically referred to as “known signal”) are also subjected to CDD.
As the amount of timing shift becomes larger, the effect of the delayed waves also grows. Hence, the receiving characteristics in the receiving apparatus compatible with the legacy system possibly deteriorate. Accordingly, in the light of compatibility with the legacy system, the smaller amount of timing shift is desired in CDD. On the other hand, as the amount of timing shift becomes smaller, the correlation among the multiple streams becomes larger, so that the separation of streams will be inadequate. Hence, in consideration of the characteristics in the MIMO system, the larger amount of timing shift is desired in CDD. Under these circumstances, CDD is applied to the legacy known signal, the legacy control signal and the MIMO control signal contained in a leading combination by using a relatively small absolute value for the amount of timing shift and, at the same time, CDD is applied to the MIMO known signal by using a relatively large absolute value for the amount of timing shift.
Under these circumstances, the inventors of the present invention came to recognize the following problems to be solved. As the amount of timing shift in a MIMO control signal, the same value as the amount of timing shift for a legacy known signal is set. As a result, CDD is implemented to a MIMO control signal contained in a leading combination by using the same value as the amount of timing shift in a legacy known signal assigned in a front part. However, the amount of timing shift in a MIMO control signal contained in the remaining combinations differs from the amount of timing shift in a MIMO known signal assigned to a front part. Accordingly, the weight estimated based on the MIMO known signal in a receiving apparatus may not be the value suitable for receiving a MIMO control signal. Then, there may be cases where error is caused in the received MIMO known signal. Generally, the MIMO control signal contains more important information than the data signal. The probability of error occurrence of signals in the remaining combinations is likely to increase.